Method of fabricating an implantable medical device by controlling crystalline structure

ABSTRACT

A method of fabricating an implantable medical device that includes deforming and heating setting a polymer construct, for use in fabricating the device, in a temperature range in which the crystal nucleation rate is greater than the crystal growth rate is disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of fabricating an implantable medical device by controlling crystalline structure.

2. Description of the State of the Art

This invention relates generally to implantable medical devices having a range of mechanical and therapeutic requirements during use. In particular, the invention relates to radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.

A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been subjected to angioplasty or valvuloplasty.

The stent must be able to satisfy several mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. This requires a sufficient degree of strength and rigidity or stiffness. In addition to having adequate radial strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of portions of the stent. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Therefore, a stent must be capable of exhibiting relatively high toughness which corresponds to high strength and rigidity, as well as flexibility.

A stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The stent can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. A pattern can be formed in a tube, for example, by laser cutting. The scaffolding is designed to allow the stent to be radially expandable. The pattern is generally designed to maintain the longitudinal flexibility and radial rigidity required of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen.

A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes a bioactive agent. Polymeric scaffolding may also serve as a carrier of a bioactive agent.

In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended.

A polymeric implantable medical device should be mechanically stable throughout the range of stress experienced during use of an implantable medical device. Unfortunately, many polymers used for stent scaffoldings and coatings are relatively brittle under physiological conditions, e.g., at body temperature. Many polymers remain relatively brittle, and hence susceptible to mechanical instability such as fracturing while in the body.

What is needed is an implantable medical device capable of satisfying mechanical requirements during desired treatment time.

SUMMARY

The invention provides a method for fabricating an implantable medical device, the method comprising: deforming a polymer construct while maintaining the polymer construct at a temperature range of from about Tg to about 0.6(Tm−Tg)+Tg; maintaining the deformed polymer construct in the deformed state at a temperature range of from about Tg to about 0.9(Tm−Tg)+Tg for a sufficient period of time to heat set the polymer construct; and fabricating an implantable medical device from the heat set polymer construct.

Further, the invention provides a method for fabricating an implantable medical device comprising: deforming a polymer construct at a first temperature greater than a Tg of the polymer at which the crystal nucleation rate of the polymer construct is greater than the crystal growth rate; maintaining the deformed polymer construct in the deformed state at a second temperature greater than the Tg of the polymer at which the crystal nucleation rate of the polymer construct is greater than the crystal growth rate for a sufficient period of time to heat set the polymer construct; and fabricating an implantable medical device from the set polymer.

Finally, the invention provides an implantable medical device comprising: a plurality of crystalline domains having crystals dispersed within an amorphous domain, the majority of the crystals being less than about 2 microns.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a three-dimensional view of a stent which is made up of struts.

FIG. 2 depicts a graph illustrating the nucleation rate and growth rate of crystals in a polymer versus temperature of a polymer.

FIG. 3 depicts a schematic representation of the microstructure of a polymer tube deformed and heat set at temperatures closer to Tm than Tg, having fewer crystals of a larger size compared to the polymeric tube in FIG. 4, which is deformed and heat set at a temperature closer to Tg than Tm.

FIG. 4 depicts a schematic representation of the microstructure of a polymer tube deformed and heat set at temperatures closer to Tg than Tm according to the invention, having fewer crystals of smaller size compared to the polymer tube in FIG. 3, which is deformed and heat set at a temperature closer to Tm than Tg.

DETAILED DESCRIPTION

The embodiments of the present invention relate to implantable medical devices and methods to control the relationship between degree of nucleation and associated crystal growth to improve mechanical properties such as strength and flexibility, as well as an extended shelf life.

For the purposes of the present invention, the following terms and definitions apply:

As used herein, “polymer construct” refers to any useful article of manufacture made of a polymer such as a semi-crystalline polymer, or blend of polymers, any useful article of manufacture made of any material that is coated with a polymer or blend of polymers. Some examples of polymer constructs include, but are not limited to, a tube, a sheet, a fiber, etc.

“Glass transition temperature,” T_(g), is the temperature at which the polymer's amorphous domains transform from a brittle vitreous state to a solid deformable or ductile state at atmospheric pressure. In other words, the T_(g) corresponds to the temperature where segmental motion starts in the polymer chains. When an amorphous or semicrystalline polymer is exposed to an increasing temperature, both the polymer's coefficient of expansion and the heat capacity increase as the temperature is raised, indicating increased molecular motion. As the temperature is raised, the actual molecular volume in the sample remains constant, and so a higher coefficient of expansion points to an increase in free volume associated with the system and therefore increased freedom for the molecules to move. The increasing heat capacity corresponds to an increase in heat dissipation through movement. Tg of a given polymer can be dependent on the heating rate and can be influenced by the thermal history of the polymer. Furthermore, the chemical structure of the polymer heavily influences the glass transition by affecting mobility.

“Toughness” is the amount of energy absorbed prior to fracture, or equivalently, the amount of work required to fracture a material. One measure of toughness is the area under a stress-strain curve from zero strain to the strain at fracture. The units of toughness are energy per unit volume of material. See, e.g., L. H. Van Vlack, “Elements of Materials Science and Engineering,” pp. 270-271, Addison-Wesley (Reading, Pa., 1989).

A brittle material is a relatively stiff or rigid material that exhibits little or no plastic deformation. As stress is applied to a brittle material, it tends to fracture at a stress approximately equal to its ultimate strength, undergoing little or no plastic deformation in the process. A polymer below its Tg tends to be brittle. In contrast, a ductile material under an applied stress exhibits both elastic and plastic deformation prior to fracture. Above its Tg, a polymer is ductile.

A fracture may be categorized as either ductile or brittle. A relatively low amount of energy is required to fracture brittle materials. Conversely, ductile materials can absorb a relatively high amount of energy prior to fracture. Therefore, ductile materials tend to exhibit higher toughness than brittle materials. Toughness is a desirable characteristic in implantable medical devices.

The contrast between brittle fracture and ductile fracture is important in relation to an implantable medical device because it is useful in characterizing the device. The mechanical limits of an implantable medical device during use can be more accurately characterized by the amount of energy absorbed by the device rather than the strength of the device material. For example, two devices may be made of different materials having the same or similar ultimate strength. However, under the same conditions, a material with a lower toughness will fail before a material with a higher toughness.

As used herein, an “implantable medical device” refers to any type of appliance that is totally or partly introduced into a patient's body, and which is intended to remain there after the procedure. Examples of implantable medical devices include, without limitation, self-expandable stents, balloon-expandable stents, stent-grafts, implantable cardiac pacemakers and defibrillators; leads and electrodes for the preceding; implantable organ stimulators such as nerve, bladder, sphincter and diaphragm stimulators, cochlear implants, artificial bone; prostheses, vascular grafts, grafts, artificial heart valves and cerebrospinal fluid shunts. Of course, an implantable medical device specifically designed and intended solely for the localized delivery of a therapeutic agent is within the scope of this invention. The implantable medical device may be constructed of any biocompatible material.

FIG. 1 depicts a three-dimensional view of a stent 100. Stent 100 includes struts 110, which can take on a variety of patterns. The structural pattern of the device can be of virtually any design. The embodiments disclosed herein are not limited to stents or to the stent pattern illustrated in FIG. 1, but instead can be applied to other stent patterns and other devices. As shown in FIG. 1, the geometry or shapes of stents vary throughout its structure.

As indicated above, an implantable medical device, such as a stent, should be capable of exhibiting relatively high strength and rigidity, as well as flexibility since devices have varied mechanical requirements during use, both before and during treatment.

An implantable medical device may be configured to degrade after implantation by fabricating the device either partially or completely from biodegradable polymers. Polymers can be biostable, bioabsorbable, biodegradable, or bioerodable. Biostable refers to polymers that are not biodegradable. The terms biodegradable, bioabsorbable, and bioerodable, as well as degraded, eroded, and absorbed, are used interchangeably and refer to polymers that are capable of being completely eroded or absorbed when exposed to bodily fluids such as blood and may be gradually absorbed and eliminated by the body.

A biodegradable device may be intended to remain in the body until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. For biodegradable polymers used in coating applications, after the process of degradation, erosion, absorption has been completed, no polymer will remain on the stent. In some embodiments, very negligible traces or residue may be left behind. The duration is typically in the range of six to twelve months.

Representative examples of polymers that may be used to fabricate, coat, or modify an implantable medical device include, but are not limited to, poly(N-acetylglucosamine) (Chitin), Chitosan, poly(hydroxyvalerate), poly(lactide-co-glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate), polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide), poly(caprolactone), poly(L-lactide-co-ε-caprolactone), poly(trimethylene carbonate), polyester amide, poly(glycolic acid-co-trimethylene carbonate), co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronic acid).

Many polymers used for stent scaffoldings and coatings are relatively brittle and susceptible to mechanical instability at biological conditions. This is particularly true for polymers with a Tg above a body temperature, such as poly L-lactide (“PLLA”), where the polymer in the stent never reaches its Tg. As a result, PLLA has relatively low fracture toughness and a relatively low degradation rate at conditions within the human body. It is important for a device to have a high fracture toughness throughout the range of stress experienced during use of an implantable medical device.

Other potential problems with polymeric stents are creep, stress relaxation, and physical aging. Creep refers to the gradual deformation that occurs in a component subjected to an applied load. Creep occurs even when the applied load is constant. It is believed that the delayed response of polymer chains to stress during deformations causes of creep behavior. Deformation stops when the initially folded chains reach a new equilibrium configuration (i.e. slightly stretched). This deformation is recoverable after the load is removed, but recovery takes place slowly with the chains retracting by folding back to their initial state. The rate at which polymers creep depends not only on the load, but also on temperature (see temperature dependence). In general, a loaded component creeps faster at higher temperatures. Long term creep in a polymeric stent reduces the effectiveness of a stent in maintaining a desired vascular patency. In particular, long term creep allows inward radial forces to permanently deform a stent radially inward.

Stress relaxation is also a consequence of delayed molecular motions as in creep. Contrary to creep, however, which is experienced when the load is constant, stress relaxation occurs when deformation (or strain) is constant and is manifested by a reduction in the force (stress) required to maintain a constant deformation.

Physical aging, as used herein, refers to the densification in the amorphous regions of a semi-crystalline polymer. Densification is the increase in density of a material or region of a material. Densification results from residual and applied stresses. As used herein, a “residual stress” includes, without limitation, the stress in a bulk polymer that is in a non-equilibrium thermodynamic state. Physical aging of semi-crystalline polymers that have glass transition temperatures (Tg) above their normal storage temperature, which, for the purposes of this invention is room temperature, i.e., from about 15° C. to about 35° C., occurs primarily through the phenomenon known as densification.

Densification occurs when a semi-crystalline polymer is cooled at a non-equilibrium rate from a temperature above its T_(g) to a temperature below its T_(g). Such is in fact normally what will occur in most industrial settings in that equilibrium cooling is very slow and would be considered economically impractical. The non-equilibrium cooling rate results in the polymer chains of the amorphous domains being trapped at non-optimal separation distances in the glassy state that forms when the temperature goes below T_(g). The chains then attempt to achieve optimal separation by coordinated localized chain motion. Although the reordering of polymer chains do not result in chain ordering, that is, the formation of lamellae and crystallites, which would constitute crystallization, the effect on the bulk properties of the polymer is similar to that obtained when crystallization occurs, that is, the modulus of the polymer increases and concomitantly the polymer becomes more brittle. Thus, densification of a polymer initially selected for its toughness and elasticity could cause in-use failure of a construct made of or coated with the polymer when the polymer ages or densifies and becomes brittle.

Some polymers, such as semi-crystalline polymers usually contain both amorphous and crystalline domains at temperatures below their melting point. Amorphous regions are those in which polymer chains are situated in an essentially random orientation. Crystalline domains are those in which polymer chains adopt an ordered orientation with segments of separate chains or of the same chain becoming essentially parallel to one another to form structures known as lamellae. Lamellae initially form from a point of nucleation, which normally is a speck of impurity in the liquid polymer. The formed lamellae then grow outward from the nucleation point to form larger, essentially spherical crystalline structures known as crystallites.

If the polymer includes inter- or inner-chain crystalline structures, (lamellae and spherulites), which are not capable of movement unless they melt the movement of polymer chains in the amorphous domains is reduced and the ability of the chains to densify is correspondingly lessened. For this reason, increasing the degree of crystallinity of a polymer reduces or even eliminates physical aging.

Increased crystallinity, however, can be an undesirable characteristic in polymer constructs in which high toughness is important, since increased crystallinity can confer increased brittleness in a polymer. Specifically, a polymer construct can be brittle in a temperature range of use of an implantable medical device. Thus, it would be desirable to fabricate a polymer construct that is sufficiently crystalline to mitigate densification, i.e., physical aging, while reducing or eliminating the changes in bulk properties that accompany increased crystallinity, such as increased brittleness. Various aspects of the present invention provide a device and a method of fabricating the device having high fracture toughness as well as a high enough crystallization to reduce physical aging.

It is well known by those skilled in the art that molecular orientation or alignment of polymer chains in a polymer is a particularly important phenomenon that strongly influences bulk polymer properties such as fracture toughness. Orientation refers to the degree of alignment of polymer chains along a longitudinal or covalent axis of the polymer chains. For example, the strength along a particular direction in a polymer is higher when there is alignment of polymer chains along the direction. Molecular orientation along the preferred direction in a polymeric material may be induced by applying stress along the preferred direction. Generally, required strength along a particular direction can be induced in polymer construct for use in fabricating an implantable medical device.

As mentioned, radial strength is an important characteristic of stents. Therefore, strength and orientation along a circumferential direction can be induced by radially expanding a tube for fabricating a stent.

Circumferential orientation may be induced through radial expansion of a formed tube. In this process, stress induces orientation of polymer chains, and crystallization is induced by stress and oriented-induced mechanisms. By deforming or radially expanding a polymer, orientation of the polymer chains are induced.

Since polymer chain alignment is a time and temperature dependent process, a highly oriented structure that is thermodynamically stable at a given temperature may not be formed instantaneously. Thus, the polymer construct may be formed over a period of time. Therefore, after deforming a polymer construct, the polymeric construct may be heat set. “Heat setting” refers to allowing aligned polymer chains in the polymer construct to equilibrate towards the induced highly oriented structure at an elevated temperature.

In a polymer construct formed in this manner, it is important that the induced orientation is maintained such that the stent can retain the increased strength due to the induced orientation. It is desirable for the stent to be stable as well as have sufficient toughness at the temperatures of use, e.g., body temperature.

The invention provides for a method and device to control crystalline structure and maintain induced strength and molecular orientation in a device. A polymeric material deformed and heat set at the temperature range according to the invention forms a greater number of smaller sized crystalline domains as compared to polymeric material deformed and heat set a temperature near Tm of the polymer. The greater number of smaller sized crystalline domains acts as net points to keep molecular orientation and material stability of the polymer. A polymeric material having a greater number of smaller sized crystalline domains produces more tie molecules, or polymer chains that link crystalline domains, compared to a small number of larger-sized crystals. A material having a greater number of smaller sized crystalline domains exhibits higher fracture toughness and less brittleness.

A material having a greater number of smaller sized crystalline domains can be formed by controlling nucleation rate and crystal growth rate, since both are functions of temperature. Nucleation rate is the rate of formation of crystals. Crystal growth rate is the rate at which existing crystals grow. The invention provides controlling the temperature while deforming and heat setting the material such that the degree of nucleation is larger than the associated crystal growth rate. In this way, a polymeric material is formed having of a greater number of smaller sized crystalline domains and thus increased fracture toughness and reduced brittleness.

FIG. 2 is a graph of the nucleation rate A and crystal growth rate B as a function of temperature for a polymer is between Tg and Tm. Polymer crystallization begins with nucleation, the formation of small crystalline domains that begin as specks of impurities in the amorphous liquid polymer. As depicted, the rate of nucleation A occurs most rapidly at temperatures near Tg. In contrast to the rate of nucleation A, crystal growth B occurs most rapidly at temperatures near Tm. At a temperature below Tg, there is no increase in crystal growth B or increase in crystallinity. As the temperature approaches Tm, crystal growth B increases logarithmically, at which point the crystals melt and the polymer chains resume an amorphous, completely random orientation.

The invention provides for deforming a polymer construct and heat setting at a temperature that allows the relatively high rate of nucleation coupled with the relatively low crystal growth rate (See R in FIG. 2 as a general indication of the temperature range at which deformation and heat setting of the polymer construct according to the invention takes place). Thus, the invention provides a method to control the relationship between degree of nucleation and associated crystal growth rate. At such temperature range, segmental motion of the polymer increases and polymer chains are able to orient and crystallize, and form a larger number of smaller-sized crystalline domains throughout the polymer construct.

During radially expansion and heat setting the polymer construct, crystallization is induced. Depending on the application of the polymer construct, a selected period of time to heat set the radially expanded polymer tube may be adjusted to achieve a given crystallinity. Further, the temperature at which the polymer construct is deformed and heat set may be varied within the temperature range according to the desired application of the polymer construct. Therefore, deformation and heat setting are performed in a temperature range that allows a larger number of small crystals, i.e., high nucleation rate and a low crystal growth rate, to achieve a desired circumferential strength and toughness for the resultant stent.

The invention provides for deforming a polymer construct at a temperature range of from about Tg to about 0.6(Tm−Tg)+Tg, and maintaining the deformed polymer construct in the deformed state at such a temperature range for a sufficient period of time to heat set the polymer construct. By “sufficient period of time” it is meant the time necessary to form an implantable medical device that can support a lumen in an expanded state for a treatment period. The resultant polymer construct yielded from this process has a crystallinity that is substantially formed of smaller crystalline domains. In addition, the number of crystalline domains in a polymer construct according to the invention increases.

Further, the polymer construct can be a blend of two or more polymers. The polymer construct may be, but is not limited to, a tube, sheet, fiber, or any other shape for use in fabricating an implantable medical device.

At a temperature of about Tg to about 0.6(Tm−Tg)+Tg, the polymer construct may be deformed and heat set at a temperature that allows a relatively greater nucleation rate A coupled with a relatively lower crystal growth rate B. Thus, the invention provides a method to control the relationship between degree of nucleation and associated crystal growth. At such temperature range, segmental motion of the polymer increases and polymer chains are able to orient and crystallize, forming a larger number of smaller-sized crystalline domains throughout the polymer construct. Consequently, physical aging, creep, and stress relaxation are reduced or curtailed if not eliminated. The relatively large number of small crystalline domains inhibits movement of chains in the amorphous domain.

During deformation of the polymer construct, crystallization is induced. Deforming and heat setting is performed at temperatures near Tm, where nucleation rate is slow and crystal growth rate is fast, result in formation of fewer crystals of a larger size. Such a structure tends to exhibit brittleness.

In contrast, the method allows formation of smaller crystals by deforming and heat setting a polymer construct in a temperature range where nucleation rate A is relatively high, as depicted in FIG. 2, and crystal growth rate B is relatively low. Deforming and heat setting in this temperature range forms more crystalline domains of smaller size. Such a structure tends to exhibit less brittleness, and more toughness.

In certain embodiments, a method of fabricating an implantable medical device includes deforming a polymer construct in a temperature range at which the crystal nucleation rate A is greater than the crystal growth rate B. In one embodiment, the crystal nucleation rate is greater than the crystal growth rate. In another embodiment, the crystal nucleation rate is substantially greater than the crystal growth rate. In one embodiment, the polymer construct is deformed while maintaining the polymer construct at a temperature range of from about Tg to about 0.6(Tm−Tg)+Tg. In another embodiment, the polymer construct is deformed while maintaining the polymer construct at a temperature where the ratio of the crystal nucleation rate to crystal growth rate is 2, 5, 10, 50, 100, or greater than 100. In some embodiments, the deformed polymer construct is maintained in the deformed state at the selected temperature for a sufficient period of time to heat set the polymer construct. For example, in one embodiment, the deformed polymer construct is maintained at a temperature range of from about Tg to about 0.9(Tm−Tg)+Tg. In another embodiment, the deformed polymer construct is maintained at a temperature where the ratio of the crystal nucleation rate to crystal growth rate is 2, 5, 10, 50, 100, or greater than 100.

The polymer construct of the implantable medical device according to the invention may have any desired crystallinity according to the application of the device. At any given crystallinity, the polymer construct has a greater number of smaller sized crystalline domains are formed according to the invention.

FIG. 3 depicts a blown up view of a portion 300 of a strut 10 (shown in FIG. 1) that has been deformed and heat set at a temperature closer to Tm than Tg, which has fewer larger-sized crystals or crystalline domains 310 dispersed in amorphous domain 320, as compared to FIG. 4, where portion 400 is deformed and heat set at a temperature closer to Tg than Tm. Portion 300 includes crystals 310 of a larger size because deformation and heat setting of polymer construct occurs at a temperature closer to Tm than Tg, where the crystal growth rate is substantially larger than the nucleation rate. Crystalline domains 310 are of a larger size, and thus, fewer crystalline domains are formed. In FIG. 3, crystalline domains 310 are larger compared to those in portion 400. Thus, the regions of the amorphous domain between crystalline domains in portion 300 are larger.

FIG. 4 depicts a blown up view of a portion 400 of a strut 110 that has been deformed and heat set at a temperature closer to Tg according to the invention. Portion 400 has a greater number of smaller crystalline domains 410, compared with portion 300 that is deformed and heat set at a temperature closer to Tm than Tg. Crystalline domains 410 and amorphous domains 420 of portion 400 are smaller in size. By deforming and heat setting the polymer construct according to the invention for a selected period of time, the size of the crystals can range from less than about 10, less than about 6, less than about 2, or less than about 1 micron.

Orientation of polymer chains is induced by deformation along the axis of deformation. Small crystalline domains 410 serve as net points to constrain polymer chains in the amorphous domain 420 of portion 400. The motion of the polymer chains is restricted through the high number of small crystalline domains 410. A greater number of smaller-sized crystals in portion 400 can drastically restrict polymer chain motion in the amorphous domain 420 while retaining desirable bulk properties of the polymer, such as toughness.

Because amorphous domains 420 of portion 400 are constrained by the greater number of crystalline domains 410, the capacity for movement in the amorphous domains 420 of portion 400 is decreased. The shorter the distance between crystalline domains 410, the more the crystalline domains are able to constrain movement of regions of the amorphous domain 420. Crystalline domains 410 are able to exert more influence on the regions of the amorphous domain 420 surrounding crystalline domains 410 because the crystalline domains constrain movement of regions of the amorphous domain 420. The crystalline domains 410 or nucleation sites serve as “tie molecules” within amorphous domain 420 that tie the crystalline domains together, thereby locking crystalline domains into place so that movement of chains in the amorphous domains of the polymer construct is reduced. For this reason, portion 400 of the invention has smaller regions of amorphous domain 420 that resist physical aging, molecule creeping, and stress relaxation of portion 400.

In addition to controlling the physical aging of the polymer construct, the dimensional stability and brittleness of a polymer construct according to the invention is controlled. Because crystalline domains or net points serve as “tie molecules” within the amorphous domain that tie the crystalline domains together, thereby forming a tighter network of crystals that lock the crystalline domains into place, the mechanical properties of the polymer construct are improved. For example, both fracture toughness and shape stability of the polymer construct is increased. Because a tighter network of crystals is formed, the possibility of fracturing is substantially reduced because cracks are forced to propagate through and around the many discrete crystals in the polymer construct. It is believed that the smaller and greater number of crystalline domains dispersed throughout the amorphous domain absorb energy due to crack propagation.

The polymer construct such as a tube, sheet, or fiber, may be deformed using methods and devices known to persons of skill in the art. For example, a polymer tube may be deformed by radially expanding and/or axially deforming the polymer construct. As discussed above, radial expansion of the polymer tube can induce circumferential molecular orientation which can increase circumferential strength and modulus or rigidity in the polymer tube. The polymer tube may be expanded radially by application of radial pressure. For example, the polymer tube may be expanded by blow molding.

As mentioned above, since the polymer tube can be expanded and heat set in a temperature range of high nucleation rate and slow crystal growth rate, orientation of the polymer chains induced by radial expansion is better maintained by the large number of small crystalline domains. The polymer construct is deformed for a sufficient period of time according to the desired application of the polymer construct.

In one embodiment, the invention provides for heat setting by maintaining the deformed polymer construct in the deformed state at a temperature range of from about Tg to about 0.6(Tm−Tg)+Tg for a sufficient period of time to heat set the polymer construct. It may be desirable to heat set the polymer construct at a temperature higher than the deformation temperature to increase the rate of the rearrangement of polymer chains to adopt to adopt a higher oriented structure. For example, in one embodiment, the polymer construct can be heat set at a temperature of about Tg to about 0.9(Tm−Tg)+Tg to allow polymer chains to adopt the higher oriented structure. The polymer may be maintained in the deformed state by maintaining a radial pressure and axial tension in the tube.

After heat setting, the polymer tube may then be cooled to below its Tg either before or after decreasing the pressure and/or decreasing tension. Cooling the polymer construct helps insure that the tube maintains the proper shape, size, and length following its formation. Upon cooling, the deformed polymer construct retains the length and shape imposed by an inner surface of the mold.

In one embodiment, the polymer construct consists essentially of semi-crystalline poly(L-lactic acid) or “PLLA”. The glass transition temperature of PLLA is between 60° C. to about 100° C. The polymer construct of PLLA may be made by deforming the polymer construct between about Tg to about 0.6(Tm−Tg)+Tg, and maintaining the deformed polymer construct in the deformed state to heat set the polymer construct at a temperature range from about Tg to about 0.9(Tm−Tg)+Tg. In some embodiments, the polymer construct consisting essentially of PLLA is from about 45% to about 55% crystalline after deforming and heat setting the polymer construct. In certain embodiments, the polymer construct consists essentially of PLLA and is made by deforming the polymer construct at a temperature in which the crystal nucleation rate is substantially greater than the crystal growth rate.

Various embodiments of the polymer construct described above may be used to fabricate an implantable medical device, such as a stent. As indicated above, a stent can be formed from a tube or a sheet rolled into a tube. A sheet or tube, for example, may by formed by various methods known in the art such as extrusion or injection blow molding.

Additionally, as indicated above, a stent fabricated from embodiments of the polymer construct described herein can be medicated with a bioactive agent. As used herein, a bioactive agent refers any substance that is of medical or veterinary therapeutic, prophylactic or diagnostic utility. Therapeutic use refers to a bioactive agent that, when administered to a patient, will cure, or at least relieve to some extent one or more symptoms of a disease or disorder. Prophylactic use refers to a bioactive agent that, when administered to a patient either prevents the occurrence of a disease or disorder or, if administered subsequent to a therapeutic agent, prevents or retards the recurrence of the disease or disorder. For the purposes of this invention, any such agent may be included in the construct that is subjected to the method so long as the conditions of the method will not adversely affect the agent.

This invention has been described in relation to certain examples of its application, such as its applicability to constructs comprising semi-crystalline PLLA. The examples are not intended nor should they be construed as limiting this invention in any manner. Those skilled in the art will recognize, based on the disclosures herein, other polymer and other constructs to which the invention herein may be applied. All such polymers and constructs are within the scope of this invention. 

1. A method for fabricating an implantable medical device, the method comprising: deforming a polymer construct while maintaining the polymer construct at a temperature range of from about Tg to about 0.6(Tm−Tg)+Tg; maintaining the deformed polymer construct in the deformed state at a temperature range of from about Tg to about 0.9(Tm−Tg)+Tg for a sufficient period of time to heat set the polymer construct; and fabricating an implantable medical device from the heat set polymer construct.
 2. The method according to claim 1, wherein the construct is a tube and deforming a polymer construct comprises radially expanding and/or axially deforming the tube.
 3. The method according to claim 1, the deformation of the polymer construct induces orientation of the molecules and increases the strength of the polymer construct along a direction of the deformation.
 4. The method according to claim 1, wherein the polymer construct comprises a semi-crystalline polymer.
 5. The method according to claim 1, wherein the polymer construct comprises poly(L-lactic acid), and deforming and heating is done at a temperature of from about 60° C. to about 100° C.
 6. The method according to claim 1, wherein the polymer construct is selected from a group consisting of a tube, a sheet, and a fiber.
 7. The method according to claim 1, wherein the set polymer construct comprises a crystalline structure having crystals at a size less than about 2 microns.
 8. The method according to claim 1, wherein the device is a stent.
 9. A method for fabricating an implantable medical device comprising: deforming a polymer construct at a first temperature greater than a Tg of the polymer at which the crystal nucleation rate of the polymer construct is substantially greater than the crystal growth rate; maintaining the deformed polymer construct in the deformed state at a second temperature greater than the Tg of the polymer at which the crystal nucleation rate of the polymer construct is greater than the crystal growth rate for a sufficient period of time to heat set the polymer construct; and fabricating an implantable medical device from the set polymer construct.
 10. The method according to claim 9, wherein the deformation of the polymer construct induces orientation of the molecules and increases the strength of the polymer construct along a direction of the deformation.
 11. The method according to claim 9, wherein the construct is a tube and deforming the polymer tube comprises radially expanding and/or axially deforming the polymer tube.
 12. The method according to claim 9, wherein the ratio of the crystal nucleation rate to crystal growth rate is greater than
 2. 13. The method according to claim 9, wherein the first and/or the second temperature is in a range of from about Tg to about 0.6(Tm−Tg)+Tg.
 14. The method according to claim 9, wherein the polymer construct comprises poly(L-lactic acid), and first and second temperature are between about 60° C. and about 100° C.
 15. The method according to claim 9, wherein the polymer construct is selected from a group consisting of a tube, a sheet, and a fiber.
 16. The method according to claim 9, wherein the heat set polymer construct comprises a crystalline structure having crystals at a size from about 2 to about 10 microns.
 17. The method according to claim 9, wherein the device is a stent.
 18. An implantable medical device comprising: a plurality of crystalline domains having crystals dispersed within an amorphous domain, the majority of the crystals being less than about 2 microns.
 19. The device according to claim 18, wherein the device is a stent.
 20. The device according to claim 18, wherein the device comprises a biostable and/or biodegradable polymer. 